The present invention provides cleaning compositions that can be used for a variety of applications including, for example, removing unwanted resist films, post-etch, and post-ash residue on a semiconductor substrate. In particular, the present invention provides cleaning compositions that are particularly useful for back-end-of-the-line operations that minimize the use of organic components.
The background of the present invention will be described in connection with its use in cleaning applications involving the manufacture of integrated circuits. It should be understood, however, that the use of the present invention has wider applicability as described hereinafter.
In the manufacture of integrated circuits, it is sometimes necessary to etch openings or other geometries in a thin film deposited or grown on the surface of silicon, gallium arsenide, glass, or other substrate located on an in-process integrated circuit wafer. Present methods for etching such a film require that the film be exposed to a chemical etching agent to remove portions of the film. The particular etching agent used to remove the portions of the film depends upon the nature of the film. In the case of an oxide film, for example, the etching agent may be hydrofluoric acid. In the case of a polysilicon film, it will typically be hydrofluoric acid or a mixture of nitric acid and acetic acid.
In order to assure that only desired portions of the film are removed, a photolithography process is used, through which a pattern in a computer drafted photo mask is transferred to the surface of the film. The mask serves to identify the areas of the film which are to be selectively removed. This pattern is formed with a photoresist material, which is a light sensitive material spun onto the in-process integrated circuit wafer in a thin film and exposed to high intensity radiation projected through the photo mask. The exposed or unexposed photoresist material, depending on its composition, is typically dissolved with developers, leaving a pattern which allows etching to take place in the selected areas, while preventing etching in other areas. Positive-type resists, for example, have been extensively used as masking materials to delineate patterns on a substrate that, when etching occurs, will become vias, trenches, contact holes, etc.
Increasingly, a dry etching process such as, for example, plasma etching, reactive ion etching, or ion milling is used to attack the photoresist-unprotected area of the substrate to form the vias, trenches, contact holes, etc. As a result of the plasma etching process, photoresist, etching gas and etched material by-products are deposited as residues around or on the sidewall of the etched openings on the substrate.
Such dry etching processes also typically render the photoresist extremely difficult to remove. For example, in complex semiconductor devices such as advanced DRAMS and logic devices with multiple layers of back end lines of interconnect wiring, reactive ion etching (RIE) is used to produce vias through the interlayer dielectric to provide contact between one level of silicon, silicide or metal wiring to the next level of wiring. These vias typically expose, Al, AlCu, Cu, Ti, TiN, Ta, TaN, silicon or a silicide such as, for example, a silicide of tungsten, titanium or cobalt. The RIE process leaves a residue on the involved substrate comprising a complex mixture that may include, for example, re-sputtered oxide material, polymeric material derived from the etch gas, and organic material from the resist used to delineate the vias.
Additionally, following the termination of the etching step, the photoresist and etch residues must be removed from the protected area of the wafer so that the final finishing operation can take place. This can be accomplished in a plasma “ashing” step by the use of suitable plasma ashing gases. This typically occurs at high temperatures, for example, above 200° C. Ashing converts most of the organic residues to volatile species, but leaves behind on the substrate a predominantly inorganic residue. Such residue typically remains not only on the surface of the substrate, but also on inside walls of vias that may be present. As a result, ash-treated substrates are often treated with a cleaning composition typically referred to as a “liquid stripping composition” to remove the highly adherent residue from the substrate. Finding a suitable cleaning composition for removal of this residue without adversely affecting, e.g., corroding, dissolving or dulling, the metal circuitry has also proven problematic. Failure to completely remove or neutralize the residue can result in discontinuances in the circuitry wiring and undesirable increases in electrical resistance.
Cleaning compositions containing dimethyl acetamide (DMAC) are used widely for removing residue from semiconductor substrates. DMAC is particularly suitable for such applications because it is highly polar, which makes it an excellent solvent for organic residues. DMAC is also desirable because it has a high flashpoint, it is water miscible, it has a low viscosity, and it is relatively inexpensive. Unfortunately, however, DMAC is classified as a toxic material in both the United States and in Europe. In this regard, DMAC has an NPFA health rating of 2 and its MSDS indicates that it is easily absorbed through the skin. Toxicity data also suggests that DMAC may be an embryotoxin and, as such, its use has been discouraged in Europe and has received extensive scrutiny in the United States and Asia. As a result, the electronic industry, for example, will not use cleaning compositions that include DMAC.
Where cleaning of semiconductor substrates comprising aluminum is concerned such as, for example, in Al BEOL (back-end-of the-line) cleaning of ashed and unashed substrates, conventional compositions typically contain 5-50% hydroxylamine, 10-80% (alkanolamine and/or a solvent), up to 30% chelating agent and water, with water being a relatively minor component. Such compositions being largely organic, however, require an additional rinsing step (i.e., an intermediate rinsing step) such as, for example, an isopropyl alcohol rinsing step prior to a final water rinse to avoid water-induced aluminum corrosion.
Therefore, there is a need in the art for a cleaning composition that is non-toxic and environmentally friendly for back-end cleaning operations including stripping photoresist and plasma ash residue such as, for example, those generated by plasma processes without suffering from the above-identified drawbacks. There is a particular need for a water-rich hydroxylamine-containing cleaning composition that has a cleaning efficiency comparable to conventional high organic content based cleaning compositions that removes etch residues while not changing the critical dimensions of the metal structures on the substrate.